Switching power converters typically have a fixed output voltage. For example, a fixed-output-voltage AC-DC switching power converter converts the AC line voltage into a DC output voltage that is regulated to be substantially constant. An example fixed-output voltage switching power converter 100 is shown in FIG. 1 regulates an output voltage (V_OUT) across an output capacitor C1. To keep this output voltage in regulation, switching power converter 100 samples the output voltage through a voltage divider such as formed by a pair of resistors R1 and R2 to produce a feedback voltage (V_FB). An error amplifier U1 compares the feedback voltage to a reference voltage produced by a reference voltage source (V_REF). A modulator U2 controls the switching of a power switch S1 responsive to the error signal to regulate the output voltage.
The resulting fixed output voltage such as 5.0 V is problematic for fast charging of modern devices. In particular, it is conventional for a switching power converter to couple to the device being charged through a standard interface such as a Universal Serial Bus (USB) interface. The USB interface includes a differential pair of signals (D+ and D−) for signaling and also provides power and ground. With regard to the delivery of power, a USB cable can only support a certain amount of current. For example, the USB 2.0 standard allows for a maximum output current of 500 mA whereas the USB 3.0 standard can support a maximum output current of 900 mA. Traditionally, the delivery of power through a USB cable used a voltage of 5.0 V. But modern mobile device batteries have relatively large storage capacities such as several thousand milliamps. The charging of such batteries, even at the increased output currents allowed in the USB 3.0 standard, will thus be delayed if the power is delivered using a 5 volt power supply voltage. This is particularly true in that the switching power supply, the cable, and the receiving device all present a resistance to the output current.
To enable a rapid charge mode in light of the output current limitations and associated losses from device resistances, it is now becoming conventional to use markedly higher output voltages over the USB cable. For example, rather than use the conventional USB output voltage of 5 V, power switching converters have been developed that support rapid charging modes using output voltages of 9V, 12V, or even 19V. The increased output voltages allow the switching power supply to deliver more power over the USB cable without exceeding the maximum output current limitations. However, many legacy devices can only except the standard 5V from a USB cable. A rapid-charge switching power supply will thus engage in an enumeration process with the device being charged to determine if the higher output voltages are supported. This enumeration may occur over the differential D+ and D− pins. Through the enumeration, the switching power converter and the enumerated device may change the USB output voltage to an increased level that is supported by the enumerated device. The result is considerably reduced charging time, which leads to greater user satisfaction.
An example fast-charge power converter 200 is shown in FIG. 2. Error amplifier U1, modulator U2, power switch S1, output capacitor C1, and the feedback voltage divider formed by resistors R1 and R2 operate as discussed with regard to fixed-output power converter 100. However, fast-charge power converter 200 includes a resistor R3 and a fast-mode switch S1 coupled between the feedback voltage input to error amplifier U1 and ground. If fast-mode switch S1 is switched on, resistors R3 and R2 are coupled in parallel such that the resistance between the feedback voltage input and ground drops. The feedback voltage will thus drop when switch S1 is switched on, which causes the error signal to increase. The modulator U2 will thus increase its modulation of the cycling of switch S1 to increase the output voltage. For example, if modulator U2 is a pulse-width modulator, it would then increase the pulse width for the cycling of power switch S1. Conversely, if the fast-mode switch S2 is switched off, the output voltage will drop in response to the sudden increase in the feedback voltage amplitude. A mode control circuit 205 controls the fast-mode switch S2 to select between the output voltage levels.
FIG. 3A illustrates a resulting on and off waveform for fast-mode switch S2 to select between the high output voltage and low output voltage modes. When the gate voltage for the fast-mode switch transistor S2 is high, the transistor turns on to select for the high output voltage mode. When the gate voltage is 0 V, the fast-mode switch transistor S2 turns off to select for the low output voltage mode. The sudden switching on of the fast-mode switch transistor S2 at a time t0 causes an abrupt drop in the feedback voltage at the input node to the error amplifier as shown in FIG. 3B. Similarly, the switching off of the fast-mode switch transistor S2 at a time t1 causes a sharp increase in the feedback voltage. These abrupt swings in the feedback voltage are undesirable. For example, the sudden drop in the feedback voltage at time t0 can trigger an under-voltage condition whereas the sudden increase in the feedback voltage at time t1 can trigger an over-voltage condition. The corresponding switching power converter would then react to false alarm conditions that are artifacts of the abrupt changes in resistance for sensing the feedback voltage during output voltage mode transitions. A waveform for the output voltages produced by the cycling of the fast-mode switch S2 of FIG. 3A is shown in FIG. 3C. The resulting sudden changes in the error signal from the switching on of the fast-mode switch S2 at time t0 produces an undesirable over-shoot and ringing of the output voltage during the transition to the high output voltage mode. This over-shoot of the output voltage may stress or harm the device being charged.
Accordingly, there is a need in the art for improved regulation of the output voltage during output voltage shifts for a power converter having multiple output voltage modes.